As a result of the recent development of electronics, there is a rapid increase in the demand for semiconductor devices such as memories and integrated circuits.
Such a semiconductor device is manufactured by doping impurities into a highly pure semiconductor substrate to impart electrical properties thereto, and forming a minute circuit on the semiconductor substrate by etching, for example.
Such operations must be performed in a chamber in a high-vacuum state to avoid the influence of dust or the like in the air. A vacuum pump is generally used to evacuate the chamber. In particular, a turbo-molecular pump, which is a kind of vacuum pump, is widely used since it involves little residual gas and is easy to maintain.
When manufacturing a semiconductor, these are many steps for making various process gases act on a semiconductor substrate, and the turbo-molecular pump is used not only to create a vacuum in a chamber, but also to discharge these process gases from the chamber. FIG. 6 is a longitudinal sectional view of such a turbo-molecular pump.
In FIG. 6, a turbo-molecular pump 100 has an inlet port 101 formed at the upper end of an outer cylinder 127. Inside the outer cylinder 127, there is provided a rotor 103 having in its periphery a plurality of rotary blades 102a, 102b, 102c, . . . formed radially in a number of stages and constituting turbine blades for sucking and discharging gas.
A rotor shaft 113 is mounted at the center of the rotor 103, and is levitated and supported in the air and controlled in position by a so-called 5-axis control magnetic bearing, for example.
Four upper radial electromagnets 104 are arranged in pairs in the X and Y axes which are perpendicular to each other and serve as the radial coordinate axes of the rotor shaft 113. An upper radial sensor 107 formed of four electromagnets is provided in close vicinity to and in correspondence with the upper radial electromagnets 104. The upper radial sensor 107 detects a radial displacement of the rotor 103 and transmits the detection result to a control device (not shown).
Based on the displacement signal from the upper radial sensor 107, the control device controls the excitation of the upper radial electromagnets 104 through a compensation circuit having a PID adjusting function, thereby adjusting the upper radial position of the rotor shaft 113.
The rotor shaft 113 is formed of a material having a high magnetic permeability (e.g., iron), and is attracted by the magnetic force of the upper radial electromagnets 104. Such adjustment is performed independently in the X- and Y-axis directions.
Further, lower radial electromagnets 105 and a lower radial sensor 108 are arranged similarly to the upper radial electromagnets 104 and the upper radial sensor 107 to adjust the lower radial position of the rotor shaft 113 similarly to the upper radial position thereof.
Further, axial electromagnets 106A and 106B are arranged with a metal disc 111 vertically sandwiched therebetween, the metal disc 111 having a circular plate-like shape and arranged at the bottom of the rotor shaft 113. The metal disc 111 is formed of a material having a high magnetic permeability, such as iron. An axial sensor 109 is arranged to detect an axial displacement of the rotor shaft 113, and its axial displacement signal is transmitted to the control device.
The axial electromagnets 106A and 106B are excitation-controlled based on this axial displacement signal through a compensation circuit having a PID adjusting function in the control device. The axial electromagnet 106A and the axial electromagnet 106B attract the metal disc 111 upward and downward respectively by their magnetic force.
In this way, the control device appropriately adjusts the magnetic force exerted on the metal disc 111 by the axial electromagnets 106A and 106E to magnetically levitate the rotor shaft 113 in the axial direction while supporting it in space in a non-contact state.
A motor 121 has a plurality of magnetic poles circumferentially arranged around the rotor shaft 113. Each magnetic pole is controlled by the control device to rotate and drive the rotor shaft 113 through the electromagnetic force acting between the rotor shaft 113 and the magnetic pole.
Further, a phase sensor (not shown) is provided near the lower radial sensor 108 for example, to detect the rotational phase of the rotor shaft 113.
A plurality of stationary blades 123a, 123b, 123c, . . . are arranged apart from the rotary blades 102a, 102b, 102c, . . . with small gaps therebetween. The rotary blades 102a, 102b, 102c, . . . are inclined by a predetermined angle from a plane perpendicular to the axis of the rotor shaft 113 in order to transfer the molecules of exhaust gas downward through collision,
Similarly, the stationary blades 123 are inclined by a predetermined angle from a plane perpendicular to the axis of the rotor shaft 113, and arranged alternately with the rotary blades 102 so as to extend toward the inner side of tie outer cylinder 127.
One ends of the stationary blades 123 are supported while being fitted into the spaces between a plurality of stationary blade spacers 125a, 125b, 125c, . . . stacked together.
The stationary blade spacers 125 are ring-like members which are formed of, e.g., aluminum, iron, stainless steel, copper, or an alloy containing some of these metals.
The outer cylinder 127 is fixed on the outer periphery of the stationary blade spacers 125 with a small gap therebetween. A base portion 129 is arranged at the bottom of the outer cylinder 127, and a threaded spacer 131 is arranged between the lower end of the stationary blade spacers 125 and the base portion 129. An exhaust port 133 is formed under the threaded spacer 131 in the base portion 129, and communicates with the exterior.
The threaded spacer 131 is a cylindrical member formed of aluminum, copper, stainless steel, iron, or an alloy containing some of these metals, and has a plurality of spiral thread grooves 131a in its inner peripheral surface.
The direction of the spiral of the thread grooves 131a is determined so that the molecules of the exhaust gas moving in the rotational direction of the rotor 103 are transferred toward the exhaust port 133.
At the lowest end of the rotary blades 102a, 102b, 102c, . . . of the rotor 103, a rotary blade 102d extends vertically downward. The outer peripheral surface of this rotary blade 102d is cylindrical, and extends toward the inner peripheral surface of the threaded spacer 131 so as to be close to the inner peripheral surface of the threaded spacer 131 with a predetermined gap therebetween,
The base portion 129 is a disc-like member constituting the base portion of the turbo-molecular pump 100, and is generally formed of a metal such as iron, aluminum, and stainless steel.
Further, the base portion 129 physically retains the turbo-molecular pump 100 while serving as a heat conduction path. Thus, it is desirable that the base portion 129 is formed of a metal having rigidity and high heat conductivity, such as iron, aluminum, and copper.
In this configuration, when the rotor shaft 113 is driven by the motor 121 and rotates with the rotary blades 102, exhaust gas from the chamber is sucked in through the inlet port 101 by the action of the rotary blades 102 and the stationary blades 123.
The exhaust gas sucked in through the inlet port 101 flows between the rotary blades 102 and the stationary blades 123 to be transferred to the base portion 129. At this time, the temperature of the rotary blades 102 increases due to frictional heat generated when the exhaust gas comes into contact with or collides with the rotary blades 102, conductive heat and radiation heat generated from the motor 121, for example. This heat is transmitted to the stationary blades 123 through radiation or conduction by gas molecules of the exhaust gas etc.
The stationary blade spacers 125 are connected together in the outer periphery and transmit, to the outer cylinder 127 and the threaded spacer 131, heat received by the stationary blades 123 from the rotary blades 102, frictional heat generated when the exhaust gas comes into contact with or collides with the stationary blades 123, etc.
The exhaust gas transferred to the threaded spacer 131 is transmitted to the exhaust port 133 while being guided by the thread grooves 131a. 
In the example explained above, the threaded spacer 131 is arranged in the outer periphery of the rotary blade 102d, and the threaded grooves 131a are formed in the inner peripheral surface of the threaded spacer 131. However, in some cases, the threaded grooves may be formed in the outer peripheral surface of the rotary blade 102d so that a spacer having a cylindrical inner peripheral surface is arranged around the threaded grooves.
Further, in order to prevent the gas sucked in through the inlet port 101 from entering an electrical component section formed of the motor 121, the lower radial electromagnets 105, the lower radial sensor 108, the upper radial electromagnets 104, the upper radial sensor 107, etc., the electrical component section is covered with a stator column 122, and the inside of this electrical component section is kept at a predetermined pressure by a purge gas.
Accordingly, piping (not shown) is arranged in the base portion 129, and the purge gas is introduced through this piping. The introduced purge gas is transmitted to the exhaust port 133 through the gap between a protective bearing 120 and the rotor shaft 113, the gap between the rotor and stators of the motor 121, and the gap between the stator column 122 and the rotor 103.
Note that the turbo-molecular pump 100 must be controlled based on individually adjusted specific parameters (e.g., a specific model and characteristics corresponding to the model). The turbo-molecular pump 100 has an electronic circuit portion 141 in its main body to store these control parameters and maintenance information such as error history, for example. The electronic circuit portion 141 is formed of electronic parts such as a semiconductor memory like EEP-ROM and a semiconductor device for the access thereto, a board 143 for mounting the electronic parts, and so on.
This electronic circuit portion 141 is accommodated in the central portion of the base portion 129 constituting the lower portion of the turbo-molecular pump 100, and is closed by a hermetic bottom cover 145.
In same cases, the process gas is introduced into the chamber at high temperature to increase reactivity. Such a process gas cooled to a certain temperature at the time of discharge may be turned into solid to precipitate a product in the exhaust system.
Such a process gas attains low temperature inside the turbo-molecular pump 100 to be turned into solid, adhering to the inner surfaces of the turbo-molecular pump 100 to be deposited thereon.
When the precipitate of the process gas is deposited in the turbo-molecular pump 100, the deposited substance narrows the flow passage of the pump, which causes deterioration in the performance of the turbo-molecular pump 100.
The above-mentioned product is likely to solidify and adhere in low-temperature portions around the exhaust port, and particularly around the rotary blade 102d and the threaded spacer 131. Conventionally, to solve this problem, a heater 147 and an annular water cooling tube 149 are wound around the outer periphery of the base portion 129 etc. and a temperature sensor 151 (e.g., a thermistor) is embedded in, e.g., the base portion 129 to keep the base portion 129 at a fixed high temperature (set temperature) by performing heating operation by the heater 147 and cooling operation by the water cooling tube 149 (hereinafter referred to as TMS (temperature management system.)
It is desirable that the set temperature of TMS is as high as possible since the product is hardly deposited at a higher temperature.
On the other hand, when the base portion 129 is set to a high temperature as stated above, the temperature of the electronic circuit portion 141 exceeds a limit if ambient temperature changes to a high temperature due to the variation in an exhaust load etc., which may destroy a storage formed of a semiconductor memory. In such a case, the semiconductor memory is broken, and control parameters and maintenance information data concerning pump start time, error history, etc. stored in the memory are cleared.
When the maintenance information data is cleared, it is impossible to judge when the maintenance check and exchange of the turbo-molecular pump 100 should be carried out. Therefore, serious problems are caused in the operation of the turbo-molecular pump 100.
Further, a pump ID (identification information) is written in the semiconductor memory. When the power source is turned on, matching between the pump ID and the control device is performed and the pump is operated based on the result. Accordingly, when the data of the pump ID etc. is cleared, the turbo-molecular pump 100 cannot be restarted.
Similarly, when the temperature of the base portion 129 becomes high, current flowing through electromagnetic windings constituting the magnetic poles increases due to the variation in an exhaust load etc., which may cause the temperature of the motor 121 to exceed an allowable temperature. In this case, the electromagnetic windings are broken and the motor stops.
Further, the mold material of the electromagnetic windings melts, and the retention force of the mold material decreases. As a result, the arrangement positions of the electromagnets are shifted, which reduces the rotational driving force of the motor or stops the rotation of the motor.
Prior patent document 1 (Japanese Patent Laid-Open Pub. No. 2002-257079) discloses a control method as a TMS control method. Specifically, in a controller of this patent document 1, a minimum set temperature and a maximum set temperature are previously set as temperature threshold values so that a heater operates only when the temperature inside the pump body is lower than the minimum set temperature and that a cooling unit operates only when the temperature inside the pump body is higher than the maximum set temperature. When the temperature inside the pump body is between the minimum set temperature and the maximum set temperature, both of the heater and the control valve are turned off. In this way, energy loss due to temperature control can be reduced.
Further, a minimum operation time is set for each of the heater and the valve so that each of the period since the heater is turned on until the heater is turned off again by the controller and the period since the control valve is opened until the control valve is closed again by the controller becomes longer than the set minimum operation time. In this way, the chattering of the heater and the control valve can be prevented.